Method of depositing a high surface area alumina film on a relatively low surface area support

ABSTRACT

A relatively low surface area honeycomb-type ceramic support is impregnated with an alumina sol containing as a soluble plasticizer at least about 5 wt. percent of a polyethylene glycol having an average molecular weight from about 200 to about 3000. The support is thereafter calcined to form a uniformly thin, high surface area alumina film thereon.

United States Patent Hoekstra Oct. 23, 1973 54] METHOD OF DEPOSITING AHIGH 3,098,044 7/1963 Glover 252 313 R SURFACE AREA ALUMINA FILM ON A3,105,053 9/1963 Cramer et al. 252/313 R 2,931,779 4/1960 White 252 313R RELATIVE LOW SURFACE AREA 3,031,418 4/1962 Bugosh 252 313 R SUPPORTInventor: James Hoekstra, Evergreen Park, 111.

Universal Oil Products Company, Des Plaines, Ill.

Filed: June 30, 1971 Appl. No.: 158,589

Assignee:

117/169 R, 62,1196, 46 CA References Cited UNITED STATES PATENTS 12/1954Heard 252/313 R OTHER PUBLICATIONS Golding Polymers and Resins VanNostrand C0,, Princeton, NI. (1959) pgs. 354 & 355.

Primary Examiner-William D. Martin Assistant Examiner-M. SofocleousAtt0rneyJames R. Hoatson, Jr. et a1' [57] ABSTRACT A relatively lowsurface area honeycomb-type ceramic support is impregnated with analumina sol containing as a soluble plasticizer at least about 5 wt.percent of a polyethylene glycol having an average molecular weight fromabout 200 to about 3000. The support is thereafter calcined to form auniformly thin, high surface area alumina film thereon.

8 Claims, No Drawings METHOD OF DEPOSITING A HIGH SURFACE AREA ALUMINAFILM ON A RELATIVELY LOW SURFACE AREA SUPPORT This invention relates toa novel method of catalyst manufacture. More particularly, thisinvention relates to a method of depositing a high surface area aluminaas a uniform thin film on a relatively low surface area ceramic support.The present invention is particularly adapted to the manufacture of arigid, unitary, catalytic element useful to catalyze the oxidation ofcarbon monoxide and unburned hydrocarbons contained in exhaust gases ofan internal combustion engine.

Certain refractory materials, for example, the various high surface arearefractory inorganic oxides such as alumina, silica, zirconia,alumina-silica, aluminazirconia, etc., are efficient supports for anynumber of catalytically active materials when utilized in a particulateform. Such refractory supports are readily manufactured and commonlyemployed in the form of powders, pills, pellets, extrudates, or otherparticulate form including micro and macro spheroids resulting fromspray-drying or dropping techniques. However, the described refractorymaterials, which embody the high surface area characteristics soessential to efficient cat'- alyst support material, exhibit a physicalinstability under conditions of thermal stress which makes themgenerally unsuitable for use as a larger unitary support structure forcatalytic components in the conversion, for example, of automobileexhaust gases.

On the other hand, the desired physical stability is realized byutilizing certain ceramic materials such as sillimanite, petalite,cordierite, mullite, zircon, zircon mullite, spodumene, magnesiumsilicates, aluminum silicates, etc., as a unitary catalystsupport.However, although such ceramic materials are characterized-by a lowcoefficient of thermal expansion a particularly desirable feature in thelarger unitary catalyst structures herein contemplated, they areinvariably also characterized by a relatively low surface areawhichmakes them generally unsuitable as an efficient catalyst supportmaterial. It has therefore been the practice to deposit a high surfacearea film or coating on the sur-' face of the ceramic material, e.g., athin, high surface area refractory inorganic oxide film or coating.

it is an object of this invention to present a novel method of applyinga high surface area alumina as a uniform thin film on a rigid, lowsurface area, unitary support. The method of this invention isparticularly directed to the manufacture of catalysts characterized by arigid, unitary or homogenous skeletal structure, frequently referred toas a monolithic structure, comprising thin laminated sheets of acorrugated ceramic material providing a plurality of adjacent, paralleland uni-directional channels therethrough, and commonlyreferred to as ahoneycomb. While the method of this invention can also be used to applyhigh surface area coatings on particulate, finely divided catalystsupports, the real advantage is in the application of high surface areacoatings to the larger, unitary support structures in the manufacture ofcatalytic elements for inclusion in an exhaust gas converter system.

In one of its broad aspects, the present invention embodies a method ofdepositing a high surface area alumina as a uniformly thin film on arelatively low surface area refractory support, and comprisesimpregnating said low surface area refractory support with an aluminasol containing at least about 5 wt. percent of a soluble organicplasticizer; and calcining the impregnated refractory support at atemperature of at least about 425C.

One of the more specific embodiments is in a method of depositing a highsurface area alumina as a uniformly thin film on a cordierite honeycombsupport which comprises impregnating said support with an aluminumchloride sol prepared by effecting a reduction in the become apparent inthe following more detailed specification. I

The alumina sols herein contemplated are such as are formed by reducingthe acid anion concentration of an aqueous solution of an acid salt ofaluminum whereby olation occurs with the formation of inorganic polymersof colloidal dimension suspended in the remaining liquid. Such sols arein contrast to a colloidal suspension of discrete, minute particles of apreformed alumina. Suitable acid salts of aluminum include aluminumchloride, aluminum sulfate, aluminum nitrate, aluminum acetate, and thelike.

Reduction in the acid anion concentration of said solution can beaccomplished in any conventional or otherwise convenient mannerLForexample, an aqueous aluminum chloride solution can be subjected toelectrolysis utilizing an electrolytic cell with a porous partitionbetween the anode and cathode whereby acid anions are removed from thecathode compartment with the formation of an alumina sol therein. Insome cases, as with an aqueous aluminum acetate solution, where theanion is sufficiently volatile, the desired reduction in anionconcentration can be effected simply by heating. A particularly suitablemethod of preparing a sol involves the use of aluminum metal as aneutralizing agent in conjunction with, for example, an aqueous aluminumchloride solution. In this instance, the salt of neutralization isitself a hydrolyzable aluminum salt subject to ultimate sol formation.

One convenient and preferred method, particularly suitable for thepreparation of an aluminum sol comprises digesting aluminum pellets orslugs in aqueous.

hydrochloric acid, and reducing the chloride anion concentration of theresulting aluminum chloride solution through use of an excess ofaluminum reactant as a neutralizing agent. The described sols arepreferably prepared to contain about 15 wt. percent aluminum, preferablyfrom about 12 to about 15 wt. percent, with an aluminum/chloride atomratio of from about l zl to about 2:1. Generally, the sols will containfrom about 8 to about 12 wt. percent chloride.

Pursuant to the present invention, the aforesaid sol further contains anorganic plasticizer dissolved therein. The organic plasticizer isselected to improve the flexibility characteristics of the sol wherebyblistering and cracking of the surface coating is minimized during theinitial drying and calcining stages of the impregnated support material,and a durable, thin, uniform film of alumina is formed thereon. Theorganic plasticizer is suitably apolyethylene glycol with an averagemolecular weight of from about 200 to about 3,000, and preferably withan average molecular weight of from about 200 to about 700. plasticizerssubstantially as described are manufactured and available under thetradename Carbowax. In any case, the plasticizer is utilized in anamount equivalent to at least about 5 wt. percent of the sol, andpreferably from about to about wt. percent.

The alumina sol may also be prepared to contain a non-ionic surfactant,suitably at least about 0.05 wt. percent. Suitable nonionic surfactantsinclude the various" and well known polyoxyethylene alkylphenols,polyoxyethylene alcohols, polyoxyethylene esters of fatty acids,polyoxyethylene mercaptans, polyoxyethylene alkylamines, polyoxyethylenealkylamides, and the like. A polyoxyethylene alcohol with an averagemolecular weight of from about 200 to about 500 is particularlysuitable. The surfactant may be included in the s l to facilitatepenetration and even distribution of the sol, for example, in thefissures or clefts common to a honeycomb support structure. Preferably,the nonionic surfactant is utilized in an amount comprising from about0.05 to about 0.5 wt. percent based on the weight of the alumina sol.

The low surface area ceramic support may be impregnated with thedescribed sol by impregnating techniques known to the art. Thus, theceramic support may be soaked, dipped, suspended, or otherwise immersedin the sol, suitably at ambient temperature conditions. The impregnationprocess may be repeated one or more times to deposit a satisfactory thinuniform coating on the surface of the ceramic support structure. In anycase, the impregnated support is subsequently calcined in an oxidizingatomsphere at a termperature of at least about 425C. to form the desiredthin uniform high surface area alumina film deposited on the ceramicsupport.

The impregnated ceramic support is subsequently calcined over a periodof from about 1 to about 5 hours at a temperature of from about 425 toabout l,l00C. In some instances it may be desirable to reduce orsubstantially eliminate the halogen associated with the alumina, andthis is readily accomplished by effecting calcination in a steam-airatmosphere, for example in an air atmosphere containing from about 20wt. percent to about 50 wt. percent steam.

The high surface area, alumina-coated, ceramic honeycomb structures ofthis invention are particularly adapted to the manufacture of a unitarycatalyst element for the conversion of exhaust gases emanating from aninternal combustion engine. Thus, the alumina-coated honeycomb supportcan be further composited with one or more catalytically active metalliccomponents in the reduced or oxidized state. In the treatment of saidexhaust gases, platinum has been shown to be a particularly effectivecatalyst. Other metallic components particularly active in this respectinclude copper oxide, and especially copper oxide in combination withone or more promoter oxides including the oxides of iron, nickel,cobalt, chromium, manganese, tin, vanadium, and the like. Still othercatalytically active materials including the reduced or oxidized form ofpalladium, tungsten, molybdenum, silver, gold, rhenium, germanium, etc.,known for their activity with respect to various catalytic processes,are readily composited with the support material of this invention.

The catalytic components are applied to the aluminacoated substrate byconventional methods which generally entail immersing the support in anaqueous solution of a precursor compound of a desired metalliccomponent, and adsorbing and impregnating the same on the high surfacearea alumina coating. For example, the support is impregnated with anaqueous solution of chloroplatinic acid, platinum chloride, ammoniumchloroplatinate, dinitrodiamino platinum, etc., and the impregnatedsupport subsequently oxidized and/or reduced to yield the platinumcomponent in an oxidized or reduced state. Precursor compounds of thecatalytic metals which decompose upon calcination to provide the oxidescan be used. These include the soluble hydroxides, carbonates, nitratesand/or organic salts of the various catalytically active metals, as wellas ammonium salts such as ammonium metavanadate, and the like.

The following example is presented in illustration of the method of thisinvention and is not intended as an undue limitation on the generallybroad scope of the invention as set out in the appended claims.

EXAMPLE I An alumina sol was prepared by digesting an excess of aluminumin aqueous hydrochloric acid under reflux conditions (98-l 15C.). Theso] contained 14.2 wt. percent aluminum in a 15:1 ratio with thechloride anion content thereof, and had a specific gravity of 1.4. About1,860 milliliters of the sol thus prepared was admixed with 465milliliters ofa polyethylene glycol plasticizer having an averagemolecular weight of about 600 (Carbowax), and with 1.52 grams ofapolyoxyethylene alcohol nonionic surfactant having an average molecularweight of about 240 (Antarox Bis-240).

A cordierite ceramic honeycomb structure weighing 447.3 grams was thenimmersed in the described alumina so] with a gentle reciprocatingmovement in the direction of the parallel channels to insure contact andan even distribution of the sol on the honeycomb surface. After about 4minutes, the impregnated honeycomb was removed and blown free of excesssol with a stream of air. The impregnated honeycomb, containing about229.2 grams of alumina sol adhered to the surface thereof, was dried forabout 2 hours in a forceddraft furnace at l50C. The dried, impregnatedhoneycomb was subsequently heated to 540C. over a A hour interval andthen calcined at 540C. for 2 hours in a stream of air containing about30 wt. percent steam. While the ceramic honeycomb initially exhibited asurface area of less than about 1 m /gm, the aluminacoated product had asurface area of about 21 m /gm.

I claim as my invention:

1. A method of depositing a high surface area alumina as a uniformlythin film on a relatively low surface area refractory support whichcomprises:

a. impregnating said support with an alumina sol containing at leastabout 5 wt. percent of a soluble polyethylene glycol (organic)plasticizer having an average molecule weight of from about 200 to about3000; and

b. calcining the impregnated support at a temperature of at least about425C.

2. The method of claim 1 further characterized in that said refractorysupport is a ceramic honeycomb structure of cordierite.

3. The method of claim 1 further characterized in that said alumina solis prepared by effecting a reduction in the acid anion concentration ofan aqueous solution of a hydrolyzable acid salt of aluminum.

4. The method of claim 1 further characterized in that said alumina solis an aluminum chloride sol prepared by effecting a reduction in thechloride anion concentration of an aqueous solution of aluminumchloride.

5. The method of claim 1 further characterized in that said alumina solis an aluminum chloride sol prepared by effecting a reduction in thechloride anion concentration of an aqueous solution of aluminum chlorideto provide an aluminum/chloride ratio of from about 1:1 to about 2:1.

6. The method of claim 1 further characterized in that said sol containsfrom about 10 to about 20 wt. percent of said plasticizer.

7. The method of claim 1 further characterized in that said sol furthercontains from about 0.05 to about 0.5 wt. percent of a polyoxyethylenealcohol surfactant characterized by an average molecular weight of fromabout 200 to about 500.

8. The method of claim 1 further characterized in that said impregnatedsupport is calcined at a temperature of from about 425 to about l,100C.

2. The method of claim 1 further characterized in that said refractorysupport is a ceramic honeycomb structure of cordierite.
 3. The method ofclaim 1 further characterized in that said alumina sol is prepared byeffecting a reduction in the acid anion concentration of an aqueoussolution of a hydrolyzable acid salt of aluminum.
 4. The method of claim1 further characterized in that said alumina sol is an aluminum chloridesol prepared by effecting a reduction in the chloride anionconcentration of an aqueous solution of aluminum chloride.
 5. The methodof claim 1 further characterized in that said alumina sol is an aluminumchloride sol prepared by effecting a reduction in the chloride anionconcentration of an aqueous solution of aluminum chloride to provide analuminum/chloride ratio of from about 1:1 to about 2:1.
 6. The method ofclaim 1 further characterized in that said sol contains from about 10 toabout 20 wt. percent of said plasticizer.
 7. The method of claim 1further charActerized in that said sol further contains from about 0.05to about 0.5 wt. percent of a polyoxyethylene alcohol surfactantcharacterized by an average molecular weight of from about 200 to about500.
 8. The method of claim 1 further characterized in that saidimpregnated support is calcined at a temperature of from about 425* toabout 1,100*C.
 3000. THE SUPPORT IS THEREAFTER CALCINED TO FORM AUNIFORMLY THIN, HIGH SURFACE AREA ALUMINA FILM THEREON.